Preheat oven to 300 degrees. Spray 12-muffin tin with no-stick cooking spray. Mash bananas in a medium mixing bowl. Add sugar, brown sugar and eggs. Mix well. Add butter. Stir in flour, whole wheat flour and soda. Add buttermilk and walnuts. Mix until just moistened. Spoon into muffin tin. Bake until muffins are brown and done, about 20 minutes. Yield: dozen muffins.